1. Field of the Invention
The present invention relates to isolated polypeptides having xyloglucanase activity and isolated nucleic acid sequences encoding the polypeptides. The invention also relates to nucleic acid constructs, vectors, and host cells comprising the nucleic acid sequences as well as methods for producing and using the polypeptides.
2. Description of the Related Art
Xyloglucan is a major structural polysaccharide in the primary (growing) cell wall of plants. Xyloglucan is believed to function in the primary wall of plants by cross-linking cellulose-micro fibrils, forming a cellulose-xyloglucan network. This network is considered necessary for the structural integrity of primarty cell walls. Another important function of xyloglucan is to act as a repository for xyloglucan subunit oligosaccharides that are physiologically active regulators of plant cell growth. Xyloglucan subunits may also modulate the action of a xyloglucan endotransglycosylase (XET), a cell wall associated enzyme that has been hypothesized to play a role in the elongation of plant cell walls. Thus, xyloglucan might play a role in wall loosening and consequently cell expansion (Fry et al., 1992, Biochem. J. 82: 821–828).
Structurally, xyloglucans consist of a cellulose-like beta-1,4-linked glucose backbone, which is frequently substituted with various side chains. The side chains are composed of xylosyl, galactosyl, fucosyl, and/or arabinosyl residues (Hayashi et al., 1989, Annu. Rev. Plant Physiol. Plant Mol. Biol. 40: 139–168). The xyloglucans of most dicotyledonous plants and some monocotyledons and gymnosperms, are highly branched polysaccharides in which approximately 75% of the glucose residues in the backbone bear a glycosyl side chain at O-6. The glycosyl residue attached to the branched glucose residue is invariably alpha-D-xylose. Up to 50% of the side chains in xyloglucans contain more than one residue due to the presence of beta-D-galactose or alpha-L-fucose-(1-2)-beta-D-galactose moieties at O-2 of the xylose residue.
Xyloglucanases hydrolyze the beta-1,4-glycosidic linkages in the backbone of xyloglucan to xyloglucan oligosaccharides. Xyloglucanases generally exhibit minor cellulolytic activity against conventional substrates such as carboxymethycellulose, HE cellulose, and Avicel (microcrystalline cellulose).
Vicken et al. have disclosed the characterization of three endoglucanases (endoglucanases I, IV, and V) from Trichoderma viride (1997, Carbohydrate Research 298: 299–310). All three endoglucanases have high activity toward cellulose or carboxymethycellulose, but endoglucanase I (belonging to family 5 of glycosyl hydrolases) has essentially no activity toward xyloglucan, while both endoglucanase IV (belonging to family 12 of glycosyl hydrolases) and endoglucanase V (belonging to family 7 of glycosyl hydrolases) have activity against xyloglucan (see Henrissat, B., 1991, Biochem. J., 280: 309–316, and Henrissat and Bairoch, 1993, Biochem. J., 293: 781–788).
WO 97/14953 discloses a xyloglucanase gene (endoglucanase II) cloned from the Aspergillus aculeatus and expressed in Aspergillus oryzae. The xyloglucanase has high xyloglucanase activity and very little cellulase activity with xyloglucanase activity in the pH range 2.5–6 and optimum activity at pH 3–4. Hasper et al., 2002, Applied and Environmental Microbiology 68: 1556–1560, disclose an endoglucanase from Aspergillus niger with major activity toward xyloglucan.
It is an object of the present invention to provide improved polypeptides having xyloglucanase activity and nucleic acid encoding the polypeptides.